DSL technology takes advantage of the fact that although a legacy twisted metallic pair (which was originally installed to provide merely a Plain Old Telephone Services (POTS) telephony connection) might only have been intended to carry signals using differential mode at frequencies of up to a few Kilohertz, in fact such a line can often reliably carry signals at much greater frequencies. Moreover, the shorter the line, the greater is the range of frequencies over which signals can be reliably transmitted (especially with the use of technologies such as Discrete Multi-Tone (DMT), etc.). Thus as access networks have evolved, telecommunications network providers have expanded their fibre optic infrastructure outwards towards the edges of the access network, making the lengths of the final portion of each connection to an end user subscriber (which is still typically provided by a metallic twisted pair) shorter and shorter giving, rise to correspondingly greater and greater bandwidth potential over the increasingly short twisted metallic pair connections-without having to bear the expense of installing new optic fibre connections to each subscriber. However, a problem with using high frequency signals is that a phenomenon known as crosstalk can cause significant interference reducing the effectiveness of lines to carry high bandwidth signals in situations where there is more than one metallic pair carrying similar high frequency signals in close proximity to one another. In simple terms, the signals from one pair can “leak” onto a nearby line (which may be carrying similar signals) and appear as noise to the other line. Although cross talk is a known problem even at relatively low frequencies, the magnitude of this effect tends to increase with frequency to the extent that at frequencies in excess of a few tens of Megahertz (depending on the length of the lines in question), the indirect coupling (e.g. from a near end of a second line to a remote end of a first line) can be as great as the direct coupling (e.g. from the near end of the first line to the remote end of the first line).
In order to alleviate the problems caused by cross talk (especially Far End Cross Talk or “FEXT” as it is known) a technology known as vectoring has been developed in which knowledge of the signals sent over crosstalking lines is used to reduce the effects of the crosstalk. In a typical situation a single DSLAM acts as a co-generator of multiple downstream signals over multiple cross-talking lines and also as a co-receiver of multiple upstream signals from the same multiple cross-talking lines, with each of the lines terminating at a single Customer Premises Equipment (CPE) modem such that no common processing is possible at the CPE ends of the lines. In such a case, downstream signals are pre-distorted to compensate for the expected effects of the cross-talking signals being sent over the neighbouring cross-talking lines such that at reception at the CPE devices the received signals are similar to what would have been received had no cross-talking signals been transmitted on the cross-talking lines. Upstream signals on the other hand are post-distorted (or detected in a manner equivalent to their having been post-distorted) after being received at the co-receiver (the DSLAM) in order to account for the effects of the cross-talk which is has leaked into the signals during their transmission.
WO2013026479 applied for by Ericsson proposes a method of transmitting signals, in such a situation (i.e. where an indirect coupling is comparable to a direct coupling for a given line), which involves transmitting signals intended for reception by a single CPE device (a first CPE device) onto both the line directly coupled to the first CPE device and onto a crosstalking line coupled only indirectly to the first CPE device (it being directly coupled to a second CPE device). A Time Division Multiplexing (TDM) method is used to enable data to be sent (in different time slots) to the two respective CPE devices (with data being sent over both wires at the same time to only one of the CPE devices at a time). In order to ensure that the two signals constructively interfere at the receiving CPE device, the same signal as sent over one line is pre-distorted (e.g. to introduce a delay and/or phase change) before being sent over the other to account for differences in the directly vs the indirectly coupled paths.
In addition, transmission mode uniqueness is not guaranteed when multiple conductors are in close proximity. In fact, it has been demonstrated that multi-mode co-existence is inevitable in multi-conductor environments. Intuitively, the average voltage potentials of the pairs at a specific frequency are very unlikely to be equal. Due to this, the voltage potential between pairs starts to move in additional differential circuits formed from multiple pairs in a similar fashion to those in twisted metallic wires pairs. These additional modes/circuits are known as phantom modes. Additionally, it is possible for signals to travel over one or more wires with reference to a fixed common ground (earthed) potential, and such modes are referred to as common modes of transmission. The presence of additional modes, e.g. common/phantom or mixed modes, allows mode conversion continuously coupling signals (often destructively) in each mode. Unlike crosstalk between pairs, signals over mode conversion crosstalk cannot be corrected or controlled without a physical access to these interfering modes. Moreover, it is worth noting that phantom modes propagate over untwisted pairs. Hence, phantoms radiate (cross-couple) higher crosstalk levels than in ordinary pairs (which are twisted). Therefore, the differential mode suffers from energy dissipation under uncontrolled multi-mode channel environment especially at high frequencies.
EP2091196 by Alcatel-Lucent provides a method to inject signals into the phantom mode formed between two Twisted Metallic Pairs (TMPs). The injected signals are the same as those sent onto one of the TMPs, but phase-rotated so that when converted and coupled into the differential mode, they interfere constructively with the signals sent directly over the respective one of the TMPs in the normal differential mode. However, EP2091196 does not consider how to exploit this technique in more general circumstances where there is more than one possible phantom mode available (i.e. where there are more than two TMPs). Furthermore, EP 2091196 does not address any power constraint implications of the arrangement.